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¨  Forces on a falling stone
¨  What is the connection of physics to chemistry and biology?
¨  How do we see the stars and planets or even airplanes as anything but streaks of light?
¨  How much steam is required to evaporate the one kg moisture?
  Do different shaped glasses sound the same when tapped with a spoon?
¨  Can an explosion on the planet Jupiter be heard on Earth?
¨  Why does the sea look blue?
¨  Levitation using weak magnets
¨  Why is the sky blue?
¨  Why do satellite orbits decay?  Do high-Earth orbit satellites need boosting?

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A stone is released into a pond from a height.  What are All the forces which act on the stone?

ANSWER from Stephanie Wong on 31 May 2007:
If it is a stone on Earth, one will have a gravitational force pulling it towards the ground.  Because Earth contains an atmosphere, the downward motion will be decreased by the frictional force due to air, in a direction opposite of gravity.  One might have also external forces, such as wind.  As the stone reaches the pond, there will also be an upward buoyant force caused by the water.  These are the forces that most people will consider when, say, they want to track the trajectory of a projectile.

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What is the connection of physics to chemistry and biology?

ANSWER from Stephanie Wong on 23 January 2007:
Biology is the study of living things.  Living things are made of atoms and molecules, which is the study of chemistry.  The fundamental phenomena regarding how atoms and molecules are governed by the laws of physics.  I would suggest going to a local library/bookstore and reading up on general science books to see how all the sciences are interrelated.  There are many interesting books out there!

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If the Earth rotates at approximately 1000 rpms, how do we see the stars and planets or even airplanes as anything but streaks of light?

ANSWER from Imagiverse on 6 September 2006:
The speed of the Earth's rotation is approximately 1,000 miles per hour depending upon the latitude where it is measured.

Think of an airplane as if it were a vehicle like a large bus driving on an elevated highway.  The plane is moving at a certain speed on this highway, the Earth is not moving out from under it.  The space shuttle or a satellite orbiting around Earth can be seen moving pretty quickly.  Depending upon the height of the orbit, satellites appear to move faster or slower or not at all.  The shuttle goes around once about every 90 minutes.  It is moving quickly but moving along with the Earth so it just looks like a distant airplane or a moving star far in the distance.  Some satellites are placed in orbits that will cause them to move with the Earth, above one point on the surface.

Stars are so far away that you can observe them and they don't appear to move at all.  Our star, the Sun, and our moon are closer and you see them appear to rise and set as the Earth spins.  I hope this helps.

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How much steam is required to evaporate the one kg moisture?

ANSWER from Imagiverse on 27 August 2006:
This sounds too specific to be something you are just wondering about.  Is it a homework question?  I think your text book will give you the information to figure this out.

I am not sure you are asking the question properly.  Do you want to know how much steam must be released to evaporate a kilo of water?  Do you want to know how much energy is required to cause this to take place?  Does it matter if you are talking about water or another liquid?  Liquid is typically measured by volume, not weight.  Do you know how much a liter of water weighs?  What about a liter of milk?  What about a water soaked piece of wood or other material, do you know how much of the weight is from the moisture?  Is the amount of energy required to dry out a log soaked in one kg of water the same as the amount of energy required to boil away 1 kg of water on your stove?

You have some variables in your question that will make a mathematical response difficult.  The first step you have in solving your problem is figuring out what you want to know.  Then take a look at your text book and find out how to solve the problem.  You can also look on the Internet but unless your choose your search words properly and know what you are looking for, you will not easily find your answer.

Good luck with this!  If you need additional assistance, please consult your professor.  If this is not a "homework" question, please resubmit it, explaining why you want to know.  Be sure you know what you want to know and phrase the question appropriately.

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If you put equal amounts of water in a circular glass and a square sided glass will they sound the same or different when you tap the glass with a spoon?

ANSWER from Imagiverse on 21 April 2006:
This is a good experiment for you to do.  However, in order for this to be a valid experiment, you are going to have to consider:

1) volume of the glass without liquid
2) thickness of the glass

A valid experiment would have to consider all other things being equal.  Two round glass of different size with the same amount of liquid are going to give you different tones.

You also need to determine how to measure the sound.  What sounds the same (or different) to you may not sound the same to someone else.

What you may want to investigate is what makes the sounds you hear when you tap a glass of water with a spoon.  How does the sound travel and resonate?  I think if you do some research, you will be able to make an educated guess and then test your theory to discover an answer for yourself.

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Can an explosion on the planet Jupiter be heard on Earth?

ANSWER from Stephanie Wong on 15 February 2006:
To "hear" something, there needs to be a medium for which sound waves to be carried on.  Think of sound waves as a set dominoes.  When a sound is made, the sound has to pass through matter (dominoes passing the "message" along the chain of dominoes).  The receiver (final domino) receives the message.  Outer space is almost a total vacuum, meaning there isn't any matter to pass along the sound waves.  If Jupiter had an explosion there would be no medium for the sound waves to pass through so the sound would not be able to reach Earth.

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Why does the sea look blue?

ANSWER from Jenny Alvarez on 1 November 2005:
The simple answer is that the ocean looks blue because the sun is shining on molecules of water creating a reflection of the pigmentations from the sky.  In some places the ocean looks green or even black because of pigmentations from floating plants and other surrounding objects.  To better understand the answer to your question, I think you might like to look at why the sky is blue as well.

For more information about the color of the ocean and the sky look at these websites:

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Is it possible to create a such system of two weak magnets (1-2 Gauss) and to make them levitate?  Please consider the fact that the magnet which will be below is very heavy, and the magnet which is above can be replaced by a very strong magnet, 500 Gauss.

ANSWER from Nicholas Blinov on 22 April 2005:
I'm not sure I completely understand your question.  In general, it is possible (the idea of using a magnetic field was already used in some (experimental) monorail transportation systems, but they used strong magnetic fields).  In your particular case you have weak magnets (their magnetic field is comparable with the magnetic field of the earth), but I don't understand why it could be a problem here.  A more serious problem is the geometry of your system.  Magnets can attract each other or repulse depending on their orientation.  So, if you have a FINITE size system, the configuration consisting of two magnets corresponding to their repulsion ("levitation") will be unstable, and they eventually will stick to each other.  So, you need to set a constraint to prevent such a situation.  For example, you put can one magnet in a pipe, and drop the second magnet from above (need to orient your magnets properly!).

Nicholas Blinov
Associate Researcher
Department of Chemistry
University of Alberta

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Why does the sky appear blue?

ANSWER from Michelle Mock on 29 April 2004:
According to Dr. Phil Plait: "The sky is blue because it is scattering blue light from the sun."  You can find his detailed explanation at:

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Why does the orbit of Earth satellites deteriorate and eventually fall into the Earth?  Will a satellite that is high enough, like the Hubble, never have to have its orbit adjusted?  Will it always circle the Earth just as it does now?

ANSWER from Stephanie Wong on 16 June 2002:
I'll give you a primer on the physics of circular motion.  A satellite in orbit must have both a tangential and normal component of its path.  Orbital motion is best defined through tangential and circular coordinates.  The tangential component is the route taken by the satellite that is tangent (or a straight line intersecting the ellipse at only one point: O ) to the line of motion.  The normal component is the motion that points in the direction of the center of curvature towards Earth.

It is logical for a satellite to have both these components, for if a satellite were launched straight up from the Earth (ignoring the rotation of the Earth) it would simply go straight up and fall back down due to gravity.  If a satellite only had a tangential motion, it would not circle the planet but veer off in a straight line into outer space.

In a perfect unperturbed orbit, a satellite would orbit the Earth indefinitely.  The rate of change of the tangential velocity balances with the normal gravitational force exerted on the object.  The satellite neither veers off nor falls out of orbit.  However external impulses, such as atmospheric friction and solar wind, decrease the tangential acceleration.  When that happens, the normal velocity downward becomes larger than the tangential velocity.  The orbit shrinks.  As the orbit gets lower and lower, there is more atmospheric friction and more slowing down.  Eventually, the orbit decays so much that it re-enters and burns up in the atmosphere and/or lands.

The Hubble Space Telescope orbits at an average altitude of 600 kilometers, which is slightly higher than the average shuttle orbit (the shuttle launched Hubble into space and also does repair missions).  This is a relatively low Earth orbit.  So, yes, Hubble's orbit will decay.  It does not have any thrusters to keep its altitude but rather has to wait for the space shuttle to give it a boost very time it does a servicing mission.

In fact, even the moon, which is about 384 000 kilometers from Earth will eventually (in millions and millions of years) fall down and crash into the Earth.

Stephanie Wong
Edmonton, Alberta, Canada

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11 July 2007

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